This invention concerns polymer materials, and more particularly polymerizable compositions that are normally liquid in uncured state, and which upon polymerization form highly flexible rubber-like polymers or elastomers. Depending on the exact formulation, such materials may find utility as sealing compounds, gasketing compounds and adhesives, although other uses are contemplated.
Rubber-like elasticity is in many respects a unique phenomenon, involving physical properties markedly different from those of low-molecular-weight solids, liquids, or gases. Typically, rubber-like materials or elastomers exhibit the following physical characteristics:
a. Ability to stretch rapidly and considerably under tension, reaching high elongation with low damping, i.e., little loss of energy as heat;
b. High tensile strength and high modulus (stiffness) when fully stretched;
c. Rapid retraction, exhibiting the phenomenon of snap or rebound; and
d. Recovery of their original dimensions fully on the release of stress, exhibiting the phenomena of resilience and low permanent set.
(See F. W. Billmeyer, Textbook of Polymer Science, John Wiley & Sons, Second Edition, 1971, page 191).
On the molecular level rubber-like materials or elastomers may be characterized by the following features:
a. Moderate to high polymer material;
b. Above its glass transition temperature T.degree.;
c. Amorphorous in its stable (unstressed) state; and
d. Containing a network of crosslinks to restrain gross mobility of its chains.
A unique feature common to elastomers is the presence of moderate to long polymer chains. These chains ordinarily are connected to one another by cross-linkages, but the preponderance of the structure typically consists of the intervening polymer chains each comprising a relatively large number of single bonds between points of cross-linkage. The process by which a network of cross-links is introduced into a polymer material to produce rubberlike elasticity is referred by as "vulcanization", although "vulcanization" and "curing" often are used synonomously with "crosslinking". Vulcanization provides many relatively weak "fix points" to the more diffuse network of strong primary bonds of a polymer material thus restraining the long-range movements of the polymer molecules, while leaving their local segmental mobility high. In a sense vulcanization may be considered as the chemical modification of a polymer material that decreases the flow of the material, increases its tensile strength and modulus, but preserves its extensibility.
Generally speaking, most elastomeric materials are prepared via a two-step process. The first step which consists of preparation of a moderate to long polymer chain, and the second step in which the moderate to long chain polymer is fabricated into the shape of the finished product. Vulcanization takes place during fabrication, i.e. the second step, and typically involves subjecting the moderate to long chain polymer material to conditions of elevated temperature, and/or elevated pressure in the presence of sulfur, an oxidizing agent or a free radical generator. A disadvantage of conventional cure-to-elastomer polymerizable materials is the requirement that the materials be made into elastomeric products only in vulcanization dedicated processing equipment.
Cure-to-elastomer polymerizable compositions based on single-component polysiloxane block copolymers offer an advantage over conventional elastomeric materials in that they can be cured in-situ, e.g., in the presence of atmospheric moisture. However, single-component polysiloxane block copolymers are somewhat thick making them difficult to apply. Moreover, single-component polysiloxane block copolymers require relatively long cure, and the resulting cured products exhibit only moderate elasticity and flexibility.
It is thus a primary object of the present invention to provide novel and improved cure-to-elastomer polymerizable compositions which overcomes the aforesaid and other disadvantages of prior art cure-to-elastomers polymerizable materials. Other objects are to provide improved cure-to-elastomer polymerizable compositions demonstrating extended pot life and ease of application, and which are capable of undergoing vulcanization in-situ. Yet other objects are to provide single-component, cold, i.e. room temperature, or moderate temperature, e.g. 100.degree. C., vulcanizable cure-to-elastomer polymerizable compositions of the aforesaid type which vulcanized products demonstrate superior elasticity and flexibility, rapid cure, and chemical and moisture resistance. Yet other objects will appear obvious and will in part appear hereinafter.
The invention accordingly comprises the processes involving the several steps in relative order of one or more steps with respect to each other, and the materials and compositions possessing the features, properties and relations of elements which are exemplified in the following disclosure and scope of application of which will be indicated in the claims.